The present invention relates to solid state shear pulverization of polymeric material, which may include thermodynamically incompatible polymers, to form without compatibilizing agents pulverized particulates that are directly melt processable as powder feedstock to shaped articles of manufacture by conventional blow molding, rotational molding, extrusion, and spray coating techniques without color streaking in the resulting articles of manufacture.
Decreasing landfill space and rapidly rising disposal costs have forced many municipalities to begin curbside recycling of post-consumer plastic (polymeric) waste. In general, plastic materials comprise approximately 20% by volume of the municipal waste stream. For example, Chem Systems, 1992, reports that municipal solid waste comprises, by weight, 48% polyethylene (PE) (27% being low density PE and 21% being high density PE), 16% polypropylene (PP), 16% polystyrene (PS), 6.5% polyvinyl chloride (PVC), 5% polyethylene terephthalate (PET), 5% polyurethane, and 3.5% other plastics.
Post-consumer polymeric waste, as opposed to industrial plastic waste, typically includes substantial quantities of plastic bottles, containers and packaging materials. Plastic bottles are molded of different polymeric materials depending upon the product they are to contain. For example, plastic bottles for water, milk, and household chemicals typically are made of high density polyethylene (HDPE), while soft drink bottles are typically made of polyethylene terephthalate (PET) with or without base caps made from high density polyethylene (HDPE). Generally, HDPE bottles account for approximately 50-60% and PET bottles account for approximately 20-30% of the bottles used by consumers. The balance of bottles, bottle caps and other containers used by consumers comprises other polymeric materials, such as low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and other resins and multi-layered materials.
Plastic packaging materials also are made of a wide variety of polymers. For example, according to Plastics Compounding, Nov/Dec, 1992, the following polymers were used in packaging material in the %""s set forth: 27% LDPE, 21% HDPE, 16% PS, 16% PP, and 5% PET.
Post-industrial plastic waste can comprise polyolefins, PS, PET and other polymeric materials used for plastic packaging. Currently, collection of plastic waste material exceeds the market demand for recycled plastic products as a result of the dearth of viable recycling technologies that are low cost and produce high quality recycled plastic products. One recycling approach has involved the high energy consuming batch grinding of commingled, unsorted mixed color plastic waste to form flake scrap material, melt processing and pelletizing the melt processed material to pellets, and extruding the pelletized plastic waste to form recycled plastic products. However, recycled plastic products made in this manner suffer from severe deficiencies that render the products unsatisfactory for many purposes and are of inferior, low value compared to products made of virgin polymeric materials. For example, these recycled plastic products exhibit inferior mechanical properties (e.g. tensile, flexural and impact strength) and inferior appearance in terms of color (dark brown or gray color) with streaking of colors within the molded product as a result of the chemical incompatibility of the different polymers present in the initial plastic waste stream and variations in the plastic waste stream composition over time.
A typical example of a low value, recycled plastic product is recycled plastic lumber having a dark brown or gray color with noticeable color streaking and inferior mechanical properties compared to components molded of virgin materials. As a result of the less than pleasing appearance, recycled plastic lumber is oftentimes painted to improve its appeal to the customer, or expensive pigments and other additives are added to the feedstock during the manufacturing process to this end. However, the cost of the recycled product is increased thereby.
Furthermore, certain melt processing techniques, such as blow molding, rotational molding, extrusion (e.g. extruded PVC pipe and profiles), and spray coating, require a plastic powder feedstock. That is, the flake scrap material is not directly melt processable to articles of manufacture by such powder feedstock-requiring melt processing techniques. To be useful as feedstock in such melt processing techniques, sorted or unsorted flake scrap material produced by batch grinding must be pelletized and then ground to powder form. The need to pelletize and grind sorted or unsorted flake scrap polymeric material prior to such melt processing adds considerably to the cost and complexity of recycling scrap plastics as well as the capital equipment expenditures required.
Conventional injection molding techniques require plastic pellets for high speed production of molded parts. Although unsorted, commingled flake scrap materials could be pelletized to provide feedstock for injection molding, the resultant molded products would suffer from the types of deficiencies discussed above attributable to polymer incompatibility.
So-called compatibilizing agents and/or reinforcing agents can be added to flake plastic scrap material comprising chemically incompatible polymers in attempts to produce a recycled plastic product exhibiting more desirable characteristics. However, addition of these agents to the plastic scrap material makes recycling more difficult and adds considerably to its cost. The Mavel et at. U.S. Pat. No. 4,250,222 relates to this type of recycling approach and is representative of the disadvantages associated with such an approach to plastic recycling.
Attempts have been made to sort commingled, post-consumer plastic scrap to overcome the polymer incompatibility problems associated with the recycling of commingled plastic scrap. To-date, HDPE and PET are recovered from plastic waste streams by recycling technologies requiring sorting of the commingled plastic materials. Sorting, however, can require use of costly techniques, such as video cameras, electronic devices, infrared detectors, and organic xe2x80x9cmarkersxe2x80x9d, to provide effective segregation of like plastics.
The high cost of sorting has greatly limited widespread use of recycling approaches that require a sorting step. In particular, collected and sorted post-consumer plastic materials are usually more expensive than the corresponding virgin plastic materials. Thus, users of plastic materials are discouraged from using sorted, recycled plastic materials.
Further, sorted plastic scrap must be subjected to batch grinding to produce flake scrap material that then must be pelletized and ground again to provide powder feedstock for blow molding, rotational molding, some extruding, spray coating and other melt processing techniques that require powder feedstock.
Even sorted plastic waste, however, can present problems in processing as a result of density and chemical differences among polymers falling in the same general class and made by different plastics manufacturers. The same polymer, for example, may have different viscosities in different products. Such differences in viscosity tend to make melt mixing of the same polymer from different products difficult and time consuming.
A study of the effect of viscosity differences on the ability to melt mix polymers was conducted by Chris E. Scott and Sandra K. Joung at the Massachusetts Institute of Technology, Department of Materials Science and Engineering. The results of this study appear in Scott and Joung, Viscosity Ratio Effects in the Compounding of Low Viscosity, Immiscible Fluids into Polymeric Matrices, Polymer Engineering and Science, Vol. 36, No. 12, June 1996 (hereinafter xe2x80x9cScott and Joungxe2x80x9d), the contents of which are incorporated herein by reference.
According to Scott and Joung, many low viscosity, immiscible fluids are difficult to incorporate into polymer matrices because of thermodynamic immiscibility and a large mismatch of melt viscosities. A model system was used in their study to determine the mechanisms and kinetics of mixing in such formulations. The model systems consisted of a series of different molecular weight polyethylenes (PE) in polystyrene (PS). The viscosity ratio (major/minor) at 180 degrees Celsius and 100/s was varied from 1.43 to 333. During the study, phase inversion of these formulations in response to compounding was observed. The phase inversion was associated with a transition from low to high mixing torque during compounding. This change was primarily due to an increase in the blend viscosity caused by the morphological transformation. The melting behavior during compounding depended on the melt viscosity of the polyethylene.
According to Scott and Joung, a critical viscosity ratio (major/minor) of 10 exists above which softening of the polystyrene, and thus mixing of the two components, was greatly retarded. Even at very low concentrations, low viscosity polyethylene can have a significant effect on the processing behavior. Effects of mixer set temperature, degree of fill, and polyethylene particle size were explored during the study. The roles of thermal conduction and mechanical energy input were evaluated in the melting regime of the process. FIG. 1A is a graph of mixing torque with respect to time for a PS/PE-D blend with a PE-D concentration of 7.8 wt % and mixer set temperatures of 140, 160, 180, and 200 degrees Celsius. The phase inversion is represented by a sudden rise in mixing torque. After the phase inversion, the mixing torque remains substantially constant. Notably, even at the high temperature of 200 degrees Celsius, it takes about five minutes for the phase inversion to occur. At the lower temperatures, it takes even longer. The study by Scott and Joung therefore demonstrates that melt mixing of polymers with a viscosity ratio (major/minor) greater than 10 is difficult and time consuming. Such polymers thus are conventionally considered to be practically incompatible.
The study by Scott and Joung also demonstrates that there is no delayed phase inversion when the polymer materials have the same viscosity (i.e., a viscosity ratio of 1) or when the viscosities are sufficiently close to one another. However, when the polymer materials have significant differences in their respective viscosities, a phase inversion is observed in response to prolonged melt mixing. The absence of a delayed phase inversion when a mixture of materials is melt processed, therefore, tends to indicate that the two materials, whether the same or different polymers, are intimately mixed with one another.
It is a primary object of the present invention to overcome one or more of the foregoing problems, and to satisfy the need in the art for a process of compatibilizing and/or intimately mixing polymers such that products of superior quality can be easily and inexpensively made by melt processing the resulting mixture.
It is another object of the invention to provide a method of processing one or more polymeric materials, such as sorted or unsorted, commingled scrap polymeric material, by solid state pulverization to produce pulverized particulates (e.g. powder) that can be directly formed to shape by powder feedstock-using melt processing techniques.
It is still another object of the invention to provide a method of processing polymeric materials, such as sorted or unsorted, commingled scrap polymeric materials, having mixed colors by solid state pulverization to produce pulverized particulates that are melt processable to a substantially homogeneous light color without color streaking or marblizing despite being produced from the mixed color scrap materials.
It is a further object of the invention to provide a method of processing polymeric materials in a manner to achieve in-situ compatibilization of different polymers present.
It is a further object of the present invention to provide a method of recycling commingled scrap polymeric materials without sortation and in a manner to achieve in-situ compatibilization of different polymers present and produce recycled polymeric particulates without the need for a compatibilizing agent.
It is still another object of the present invention to provide a method of recycling commingled, mixed-color scrap polymeric materials without sortation and in a manner to produce recycled polymeric particulates that are melt processable to homogeneous light color without color streaking or marblizing.
It is still another object of the present invention to provide solid state pulverized polymeric particulates that are suitable as powder feedstock for melt processing by blow molding, rotational molding, some extruding, spray coating and other powder feedstock-using melt processing techniques.
It is still a further object of the present invention to provide solid state pulverized polymeric particulates that are melt processable to a homogenous light color, despite being produced from mixed-color polymers.
It is still a further object of the invention to produce articles of manufacture, including molded parts and coatings, made from the aforementioned solid state pulverized polymeric particulates.
To achieve these and other objects, the present invention provides a process of compatibilizing polymer materials. The process comprises the steps of providing at least first and second polymer materials; effecting a chemical change in the polymer materials by applying mechanical energy thereto through solid state shear pulverization in the presence of cooling, and discharging particles produced by effecting the chemical change. The cooling is sufficient to maintain the polymer materials in a solid state during the pulverization. The pulverization generates a particulate mixture of the polymer materials which exhibits a more stable microstructure when annealed than mixtures produced by melt mixing of the polymer materials. The present invention also provides a product by this process.
According to another aspect of the present invention, a process of intimately mixing polymer materials is provided. The process comprises the steps of providing at least first and second polymer materials, applying mechanical energy to the polymer materials through solid state shear pulverization in the presence of cooling, to effect more intimate mixing of the first and second polymer materials than would be provided by melt mixing of those materials, and discharging particles produced by applying the mechanical energy. The cooling is sufficient to maintain the polymer materials in a solid state during the pulverization. The present invention also provides a product by this process.
According to yet another aspect of the present invention, a process of mixing polymer materials is provided. The process comprises the steps of providing at least first and second polymer materials, performing solid state shear pulverization on the polymer materials such that particles of the polymer materials which are produced by the pulverization exhibit no substantial time delay before phase inversion when subsequently subjected to melt processing, cooling the polymer materials sufficiently during the solid state shear pulverization that the polymer materials remain in a solid state, and discharging the particles produced by the solid state shear pulverization. The present invention also provides a product by this process.
Still another aspect of the present invention is provided by a process of mixing and compatibilizing polymer materials. The process comprises the steps of providing at least first and second polymer materials, performing solid state shear pulverization on the polymer materials while keeping such polymer materials in a solid state, and discharging particles produced by the solid state shear pulverization. The pulverization is performed so as to effect a chemical change in the polymer materials and to effect more intimate mixing of the polymer materials than would be provided by melt mixing of the polymer materials, resulting in a mixture of the polymer materials which exhibits a more stable microstructure than other mixtures generated by melt mixing the polymer materials. The present invention also provides a product of the method.
According to yet another aspect of the present invention, a process of recycling commingled polymeric feedstock is provided. The process comprises the steps of providing commingled polymeric feedstock made from materials having different characteristics, effecting a chemical change in the commingled polymeric feedstock by applying mechanical energy thereto through solid state shear pulverization in the presence of cooling, and making a product from particles generated by effecting the chemical change. The product is microstructurally stable as a result of the chemical change. The cooling is sufficient to maintain the polymer materials in a solid state during the pulverization.
The present invention also provides, in another aspect, a method of making polymeric particulates (e.g. powder) wherein sorted or unsorted, commingled polymeric scrap material, virgin polymeric material and mixtures thereof are supplied to extruder screw means rotated to transport the material along the length thereof and in the solid state convert the material to pulverized particulates (e.g. powder) that are melt processable directly by conventional blow molding, rotational molding, extrusion, spray coating and other melt processing techniques requiring a powder feedstock. This avoids the need for and costs associated with flake pelletizing and pellet grinding operations heretofore required.
The solid state pulverized particulates also are melt processable by conventional molding, extruding, spray coating and the like to form articles of manufacture having a substantially homogenous color appearance without color streaking or marbleizing. This color homogeneity is achievable regardless of whether the particulates include mixed color polymeric material of the same or different composition. This avoids the need for the addition of pigments and/or compatibilizing agents to the feedstock and the need to paint the molded or extruded product to hide unpleasant colors and color streaking.
The present invention provides in another aspect a method of making polymeric particulates wherein polymeric material, such as unsorted polymeric scrap material, comprising two or more thermodynamically incompatible polymers is supplied to extruder screw means rotated to transport the material along the length thereof and subject the material to solid state pulverization and in-situ polymer compatibilization.
In-situ polymer compatibilization is evidenced, in one instance, by the resulting pulverized polymeric particulates exhibiting a thermogram different from that of the precursor unpulverized material. For example, the pulverized particulates of the invention exhibit a melting peak and/or crystallization peak quite different from that (those) of the unpulverized material. Moreover, molded articles produced from the pulverized particulates of the invention exhibit increased tensile strengths and lack of delamination upon breaking in mechanical testing, this being a further indication of in-situ polymer compatibilization.
As further evidence of polymer compatibilization, hen the pulverized particulates are melt processed, the resulting products have a stable microstructure. That is, recrystallization of the product remains inhibited, even after the product is annealed for 2 hours. In addition, the glass transition temperature of the product remains substantially the same before and after annealing for two hours. Such a stable microstructure provides further evidence that the aforementioned process achieves polymer compatibilization.
In practicing the present invention, the polymeric scrap material and/or virgin material can include thermoplastics, polymer blends, polymer alloys, thermosets, elastomers and other polymeric materials. Typically, the polymeric material is comminuted to flake form by grinding, chopping or shredding using conventional equipment prior to pulverization. The pulverization process uses as scrap feedstock a material that is in a physical form (e.g. comminuted flakes) commonly available from scrap collections and municipal recycling centers.
Also, in practicing the present invention, the polymeric material can be heated during the initial stage of the pulverization operation depending upon the make-up (composition) of the feedstock followed by cooling during subsequent stages of the pulverizing operation to maintain proper temperature control for solid state pulverization, in-situ polymer compatibilization and production of desired powder size. Preferably, however, the polymeric material is only subjected to frictional heating during the initial stage of the pulverization operation by engagement with the rotating screws. That is, solid state shear pulverization of the polymeric material preferably is conducted without heating of the material by any external extruder barrel heating device. Temperature control of the polymeric material during the pulverization operation is thereby facilitated to reduce degradation of the polymers and dye materials used with the feedstock polymers. Energy consumption during the pulverization operation also is reduced.
The present invention provides in still another aspect a method of making an article of manufacture having a substantially homogenous color from mixed-color polymeric material, such as sorted or unsorted, comminged polymeric scrap material. In this embodiment of the invention, mixed-color polymeric material of the same or different composition is supplied to extruder screw means rotated to transport the polymeric material along the length thereof to subject the material to solid state pulverization to form pulverized particulates The pulverized particulates are molded, extruded or otherwise melt processed to form a substantially homogeneously colored shape characterized by the absence of color streaking and marblizing, despite the particulates originating from mixed-color polymeric material. Typically, the pulverized powder is processable to a substantially homogenous pastel color tone corresponding to a dominant color of a particular scrap component in the feedstock.
The present invention also provides solid state pulverized particulates produced from scrap polymeric material and/or virgin polymeric material wherein the particulates are suitable as powder feedstock, without conventional melt pelletizing and pellet grinding, for direct melt processing to shape using blow molding, rotational molding, some extrusion, spray coating, and other powder feedstock-using techniques.
The present invention further provides solid state pulverized polymeric particulates comprising two or more otherwise thermodynamically incompatible polymers produced from commingled, unsorted polymeric scrap materials and/or virgin materials. The polymers are in-situ compatibilized by solid state shear pulverization as evidenced by one or more different thermogram characteristics between recycled particulates of the invention and unpulverized polymeric material. Typically, the solid state pulverized particulates exhibit enhanced reactivity as compared to the unpulverized polymeric material.
Moreover, the present invention provides solid state pulverized polymeric particulates that exhibit, pulverized and as-melt processed, a substantially homogenous color despite being pulverized from mixed-color scrap material.
Articles of manufacture and powder coatings produced from the solid state pulverized particulates of the present invention exhibit mechanical properties generally superior to those exhibited by like processed flake polymeric material of the same composition depending on the polymer components involved. Importantly, they also exhibit a substantially homogeneous color characterized by the absence of color streaking or marblizing. Typically, the articles of manufacture exhibit a substantially homogeneous pastel color tone corresponding to a dominant color of a scrap component in the polymeric feedstock. Importantly, the recycled, pulverized particulates of the invention made from mixed-color polymeric feedstock can be used in molding a plurality of articles of manufacture that exhibit substantially the same homogeneous pastel color from one article to the next. In contrast, a mixture of unpulverized flake polymeric material of like composition and mixed color produces molded articles exhibiting inconsistent colors from one molded article to the next.
The present invention is advantageous in that the pulverized particulates are suitable for direct use as powder feedstock for powder feedstock-using melt processing techniques without the need for pelletizing and pellet grinding operations. Moreover, commingled scrap polymer materials, virgin polymeric materials and mixtures thereof can be processed in a manner to achieve in-situ compatibilization of different polymers in a once-through pulverization operation without the need for a compatibilizing agent and without sortation in the case of commingled scrap feedstock. The pulverized particulates may be mixed with fillers, reinforcing agents, flame retardants, antioxidants and other additives commonly used in the plastics industry if desired.
Moreover, the present invention is advantageous in that sorted or unsorted, commingled mixed-color polymeric materials and/or virgin polymeric materials can be pulverized as polymeric particulates that are melt processable to substantially homogeneous light color without the color streaking or marblizing heretofore experienced using other recycling procedures.
The present invention can provide a high value, low cost recycled particulates product, as well as products molded or otherwise melt processed therefrom, thereby increasing utilization of available plastic scrap.